The invention relates to fused silica glass. More particularly, the invention relates to fused silica glass having high levels of compositional and refractive index homogeneity. Even more particularly, the invention relates to fused silica glass having high levels of homogeneity and low levels of dopants.
High purity fused silica glass is used in the manufacture of precision lenses that operate in the ultra violet (UV) region of the radiation spectrum. Such lenses are used in the semiconductor area, and are particularly used in the projection optics train in steppers or scanners used in photolithography. In this application, the glass must possess high purity, very high homogeneity of the refractive index (also referred to herein as “refractive index”), and high transparency and stability against photo-induced changes in the UV region of the spectrum.
The purity, stability, high transparency, and index homogeneity of the fused silica glass is dependent on the level of dopants in the glass. Such dopants may be introduced during various processing steps. For example, hydroxyl (OH) dopants are typically introduced during formation of silica soot by flame hydrolysis. Halogens (fluorine, chlorine, bromine, iodine) are used to “dry” or remove water and OH from silica soot. Other dopants, such as molecular hydrogen (including molecular hydrogen enriched with certain isotopes, such as deuterium), and deuteroxyl (OD) groups are intentionally added to the fused silica during processing. Both dynamic properties and static properties of the fused silica glass are affected by the concentration levels of such dopants.
It is anticipated that future generations of optics used in lithography will require further reductions in the concentration levels of such dopants in fused silica glass optical members and higher levels of index homogeneity.